In general, magnetic resonance imaging (MRI) examinations are based on the interactions among a primary magnetic field, a radiofrequency (RF) magnetic field and time varying magnetic gradient fields with gyromagnetic material having nuclear spins within the subject of interest. Certain gyromagnetic materials, such as hydrogen nuclei in water molecules, have characteristic behaviors in response to external magnetic fields. The precession of spins of these nuclei can be influenced by manipulation of the fields to produce RF signals that can be detected, processed, and used to reconstruct a useful image.
The magnetic fields used to generate images in MRI systems include a highly uniform, static magnetic field that is produced by a primary magnet. A series of gradient fields are produced by a set of gradient coils located around the subject. The gradient fields encode positions of individual plane or volume elements (pixels or voxels) in two or three dimensions. An RF coil is employed to produce an RF field. This RF field perturbs the spins of some of the gyromagnetic nuclei from their equilibrium state, causing the spins to process around the axis of their equilibrium magnetization. The spins return to the equilibrium state after the RF field is withdrawn. During this process, RF fields are emitted by the spinning, processing nuclei and are detected by either the same transmitting RF coil, or by a separate coil. These signals are amplified, filtered, and digitized. The digitized signals are then processed using one or more algorithms to reconstruct a useful image.
MRI is sensitive to many of the gyromagnetic nuclei in the human body, such as the hydrogen nuclei in water, fats, proteins, and the like. Therefore, most tissues within a patient are readily imaged. However, for some patients, such as those having metal implants resulting from arthroplastic procedures (e.g., knee replacement, hip replacement), MRI imaging can be problematic. For example, the metal used for some implants can cause significant perturbations in the static magnetic field. This magnetic field perturbation can lead to complications with slice selection as well as frequency encoding processes, which can result in signal contamination and image artifacts. Even though the magnetic field perturbation is typically limited to the vicinity of the implant, many complications resulting from arthroplasty arise within this area. Accordingly, the presence of a metal implant can be prohibitive of the diagnosis of such complications using MRI.
Some techniques have been developed for acquiring sections around the metal implant at discrete frequency offsets that account for the field perturbations. The sections are then combined to generate an image of the tissue surrounding the metal implant. Typically, such techniques may use at least 20 different sections, referred to as “frequency bins,” to create a single image. Capturing these bins can sometimes take 20 minutes or longer. Moreover, a patient must remain still during the duration of the acquisition, which, as the acquisition becomes longer, can lead to blurred images resulting from patient movement. To counteract these long-duration acquisition sequences, certain techniques have also been developed for acquiring less than all of the information normally utilized for such image reconstruction, requiring that the absent data be estimated in some way for proper, high quality image creation. However, current techniques for such data acquisition and estimation are often inadequate or subject to further improvement.